Abstract

Pure iron and aluminum powders were mixed in the equiatomic ratio and mechanically alloyed in
a high-energy ball mill for different times. Structure refinement of x-ray powder diffraction data was
performed to study the structural transformations induced by mechanical and subsequent thermal
annealing treatments. The mechanical alloying (MA) process induces a progressive dissolution of
aluminum phase into the bcc iron phase. After 32 h of MA a single-phase Fe(Al) bcc extended solid
solution, with lattice parameter a0 = 2.891 Å, average coherent domain size <D> ≈50 Å, and lattice strain 0.5%, was observed. The annealing of the specimens after MA up to 8 h favored the
aluminum dissolution in a-iron and the precipitation of the Al5Fe2 phase, whereas a nanostructured
B2 FeAl intermetallic compound was observed in the annealed samples which were previously
milled for 8, 16, and 32 h. In the same specimens a minority cubic phase Fe3AlCX,
anti-isomorphous with perovskite, derived from contamination of ethanol and introduced in the steel
vial as a lubricant agent, was also observed. Anelasticity measurements have shown the occurrence
of two main transient effects during the first thermal run. The first one occurring at 500 K in all
mechanically alloyed specimens was attributed to thermally activated structural transformations,
whereas the second at about 700 K was attributed to a magnetic order–disorder transition. During
the second run of anelasticity measurements a relaxation peak P1 in the nanostructured B2 FeAl
intermetallic compound, attributed to grain-boundary sliding mechanisms and with an activation
energy of 1.8 ± 0.2 eV was observed. In specimens milled for 8–32 h a second small peak P2 at the
low-temperature tail of the P1 peak was observed and tentatively attributed to a Zener-type
relaxation.